Typical refrigerant chillers basically comprise a compressor, condenser, expansion device and an evaporator. Within the evaporator, vaporizing refrigerant cools a supply of water that is then circulated through a network of heat exchangers to meet the cooling demand of rooms or other areas of a building.
As the cooling demand varies, the flow rate of the water might be adjusted according. Doing so, however, can make it difficult to control the chiller's response in providing the evaporator with appropriate cooling capacity because the chiller's controller might not be aware of the water's rate of flow. The goal is to maintain the temperature of the water as it leaves the evaporator at a desired target temperature (e.g., 35° F.). Without knowing the flow rate of the water, the chiller might overcorrect at low water flow rates or respond too sluggishly at higher flow rates.
To address this problem, a flow meter could be added to the water circuit; however, such meters can be rather expensive. Alternatively, water pressure sensors upstream and downstream of the evaporator could be used to help determine the approximate flow rate through the evaporator, but the accuracy of such a method can vary depending on the total water head and whether the physical condition of the evaporator remains constant over years of use. The design of the evaporator and the actual flow rate of the water can also affect the accuracy of measuring flow rate based on the pressure drop across the evaporator.